This invention relates to filters and primarily but not exclusively to the field of filtration of lubricating oil used in internal combustion engines, but is not restricted to such use and may be utilised in other contexts.
It is conventional, in the filtration of lubricating oil used in internal combustion engines, to provide a so-called full-flow filter unit. This receives oil under pressure from the usual oil flow circuit of the engine and the flow of oil is directed through a filter unit having flow paths of such small dimensions that particles of foreign matter, especially carbon, are trapped by the filter unit, whereby the oil returns to the system in a considerably cleaner state. In order to obtain adequate treatment of the total quantity of oil available in the lubricating system of the engine, it is necessary for practical purposes to ensure that there is a relatively rapid flow of oil through the filter unit, say of the order of 2 gallons per minute. Such a rapid flow, with filtration down to say 25 microns, hereinafter referred to for convenience as "full-flow", is not compatible (in a filter unit of a practicable small size) with filtration in a filter unit of very small porosity, say of the order of 1 micron, hereinafter called for convenience "micron" filtration. Nevertheless, as there is a considerable content of such very small particles in engine oil, it is desirable to be able to remove such particles by micron-size filtration if at all possible.
British Patent Specification No. 1236805 (Russo) described the use of a paper tissue roll as a filtering medium. As will be discussed below, this is a medium suitable for micron filtration of lubricating oil but the specification shows only a full-flow filter based on this material. This would cause excessive back pressure and insufficient flow.
British Patent Specification No. 965328 (Filterwerk) shows a filter unit containing a coarse filter and a "fine filter" arranged in series for full-flow of engine oil. Accordingly it may be deduced that either the "fine filter" is not a micron filter or the filter unit will suffer from the same problems as that of Russo. A by-pass valve is provided enabling oil to by-pass the fine filter if that filter blocks. Normally the by-pass is closed by a spring loaded valve and oil does not flow through the by-pass in normal use.
It has been proposed to provide a micron filter to operate in conjunction with a conventional engine oil filter. This micron filter was a separate unit and meeded to be connected by special drillings made to oil galleries of an engine or by T-connectors inserted in oil lines. Each engine required a different connection technique and the filter was not sufficiently convenient in use to be successful. Also as the arrangement was parallel, unfiltered oil was liable to be presented to the micron filter causing this to become clogged sooner than necessary.
A coarse filter and fine filter arranged in parallel in a single housing has been proposed but this necessarily results in unfiltered oil being presented to the fine filter.
It would accordingly be desirable to provide a filter unit with which a full-flow system may be converted in a quick, simple and inexpensive manner to a system which ensures that a proportion of the total filtered oil flow is always undergoing finer, e.g. micron-size filtration.